For general duplex schemes adopted in a cellular system, a Time Division Duplex (TDD) mode refers to transmitting uplink/downlink signals in different periods of time over the same operating frequency band in the uplink/downlink, where there is a Guard Period (GP) between the uplink and the downlink; and a Frequency Division Duplex (FDD) mode refers to transmitting uplink/downlink signals concurrently over different frequency carriers in different operating frequency bands in the uplink/downlink, where there is a Guard Band (GB) between the uplink and the downlink.
The frame structure of a Long Term Evolution (LTE) TDD system is somewhat complicated. As illustrated in FIG. 1, a radio frame with a length of 10 ms includes 10 sub-frames in total including special sub-frames and normal sub-frames, where each sub-frame is 1 ms in length. The special sub-frame includes three time slots: a Downlink Pilot Time Slot (DwPTS), a GP (a Guard Period between the downlink and the uplink) and an Uplink Pilot Time Slot (UpPTS). The normal sub-frames include uplink sub-frames and downlink sub-frames for transmitting uplink/downlink control channels, traffic data, etc. Particularly a radio frame can be configured with two special sub-frames (in the sub-frames 1 and 6) or can be configured with one special sub-frame (in the sub-frame 1). The sub-frame 0 and the sub-frame 5, and the DwPTS(s) in the special sub-frame(s) are usually used for downlink transmission, the sub-frame 2 and the UpPTS(s) in the special sub-frame(s) are usually used for uplink transmission, and the remaining sub-frames can be configured for uplink transmission or downlink transmission as needed.
Uplink/downlink signals are transmitted in different sub-frames over the same frequency resources in the uplink/downlink in the TDD system. In common TDD systems including a 3G Time Division-Synchronized Code Division Multiple Access (TD-SCDMA) system and a 4G TD-LTE system, uplink and downlink sub-frames are divided statically or semi-statically, and the common practice is to determine the proportion of uplink to downlink sub-frames according to the cell type and the rough traffic proportion and keep the proportion unchanged in the network planning process. This is a simple but working practice in the context of large coverage by a macro cell. However an increasing number of low-power base stations such as a pico cell, a home NodeB, etc., have been deployed for small local coverage along with the advancement of technologies, and the number of users is small and the traffic demand of the users varies significantly in these cells, thus resulting in a dynamically varying proportion of uplink to downlink traffic as needed in the cells.
In the case that a plurality of cells are deployed in the network, different cells in the traditional TDD network use the same uplink/downlink configuration, so a base station or a UE is subjected to interference from an adjacent cell in an uplink sub-frame or a downlink sub-frame as illustrated in FIG. 2A or FIG. 2B:
First type of interference from the adjacent cell: downlink reception of a UE in the current cell is subjected to interference from a downlink signal of a base station in an adjacent cell in a sub-frame in which both of the two cells perform downlink transmission; and
Second type of interference from the adjacent cell: reception of an uplink signal from a UE by a base station in the current cell is subjected to interference from an uplink signal of a UE in an adjacent cell in a sub-frame in which both of the two cells perform uplink transmission.
Also in the case that a plurality of cells are deployed in the network, if adjacent cells are configured with different proportions of uplink to downlink sub-frames, then interference across time slots as illustrated in FIG. 3 may arise. In FIG. 3, a macro cell transmits a downlink signal in a time slot in which a pico cell receives an uplink signal so that both of the two types of interference may arise between the two cells.
In the dynamic TDD system, there are sub-frames with a fixed transmission direction and sub-frames with a variable transmission direction. In each variable sub-frame, the transmission direction of an adjacent cell is flexibly variable, and the current cell may have more than one adjacent cell producing strong interference, so the types of interferences to the current cell may be different from each other. Extremely in each variable sub-frame, the interferences from the adjacent cells to downlink transmission in the current cell are different from each other. In different downlink sub-frames of the dynamic TDD system, different interferences from the adjacent cells may result in the significant difference in real channel conditions, and DL CSI measured in one downlink sub-frame may not be applicable to another downlink sub-frame in a different adjacent-cell interference condition. For example, CSI measured in a fixed downlink sub-frame is not applicable to a variable downlink sub-frame, and CSI measured in one variable downlink sub-frame is not applicable to another variable downlink sub-frame. In view of the reasons described above, the solution for configuring channel measurement and feedback currently adopted in the LTE Rel-8/9/10 is not applicable to the dynamic TDD system.
In summary, a method for configuring channel measurement and DL CSI feedback is absent in the dynamic TDD system.